Make the merges consumer before the events consumer narrows, from the old one's ack floor, so no merge is lost to the change (issue 973)

internal/broker/jetstream.go

@@ -343,6 +343,9 @@ func (j *JetStream) EnsureConsumer(c Consumer) error {
 		if c.FromNow {
 			want.DeliverPolicy = nats.DeliverNewPolicy
 		}
+		if from := j.ackFloorOfTheConsumerItTakesOver(c); from > 0 {
+			want.DeliverPolicy, want.OptStartSeq = nats.DeliverByStartSequencePolicy, from
+		}
 		if _, err := j.js.AddConsumer(c.Stream, want); err != nil {
 			return fmt.Errorf("creating consumer %s on %s: %w", c.Name, c.Stream, err)
 		}
@@ -352,6 +355,26 @@ func (j *JetStream) EnsureConsumer(c Consumer) error {
 	}
 }
 
+// ackFloorOfTheConsumerItTakesOver is the stream sequence a consumer made now starts at so as to lose nothing the
+// consumer it takes over from (Consumer.TakesOverFrom) has not acknowledged: that consumer's ack floor plus one, when
+// it exists and still filters this one's subject; zero otherwise.
+func (j *JetStream) ackFloorOfTheConsumerItTakesOver(c Consumer) uint64 {
+	if c.TakesOverFrom == "" || len(c.Filters) != 1 {
+		return 0
+	}
+	have, err := j.js.ConsumerInfo(c.Stream, c.TakesOverFrom)
+	if err != nil {
+		return 0
+	}
+	filters := append([]string{have.Config.FilterSubject}, have.Config.FilterSubjects...)
+	for _, f := range filters {
+		if f == c.Filters[0] {
+			return have.AckFloor.Stream + 1
+		}
+	}
+	return 0
+}
+
 // ConsumerState is what a reset says about a consumer, before and after.
 type ConsumerState struct {
 	DeliverPolicy string